Singe cylinder: https://www.youtube.com/watch?v=CXvYDukyVZk
Flat twin: https://www.youtube.com/watch?v=BERMDl2_mkg
Inline 5: https://www.youtube.com/watch?v=2sBuIyi753A

After the radial (https://www.thingiverse.com/thing:3224004) I wanted a couple of improvements,

• modular design (inline and/or flat twin)
• different crank
• more efficiënt, the goal is to make it run on lung power ;)
• mostly parametric design

This design can be built as a single, a flat twin, or any multiple of those. You just need to print the cylinder 21mm apart so it's fused in your slicer. You should be able to make any inline or flat design. My goal is to make an inline 5, that's why the crank has a 72 degree offset.There is also a 90 degree offset crank for a twin/four.

Tips and requests are welcome ;)

WARNING, although I can run them at rediculous high pressures for something made of the cheapest PLA available, it IS dangerous. If your's doesn't explode it will propably make lots of torque like mine does and CAN hurt you, be careful. Wear gloves, glasses and hearing protection.

Part list for a flat twin

• some PLA
• 2 m2 10mm bolts for the pistons
• 2 m2 10mm bolts for the rockers
• 10 m2 10mm bolts for the heads
• 2 m2 10mm bolts for the cam follower
• 2 4,2mm m2 thread for the cam follower bearing (optional)
• 1 2mm * 14mm steel rod for the crank
• 4 m3 10mm bolt for the crank cases
• 2 6mm airsoft bullet, or print the valve..
• 2 2mm * 15mm steel rod for the valves (drill the airsoft bullet), experiment with the length.
• 2 2mm * 33mm steel rod as pushrod, experiment with the length
• 1 m3 hex bolt and nuts for the propellor
• 1 m3 bolt for the other side of the crank
• 2 O-rings 4,5mm inner, 8,5mm outer diameter, 1.85 mm thick (sort of, for the valve seal)

• 2 15x6x5mm crank ball bearing

• you need to trim the pushrod configuration so it has a little play (0,1mm)
• you need to plug the front holes, print extra nipples and melt them shut, or get 2 small m5 bolts
• all m3 holes can be tapped.
• i drilled all m2 holes to 1,5mm and then forced a sharpened m2 thread in there, letting it cool inside. This gave a better result then tapping m2.

changelog ;)

2019-04-23

• I've removed all files and uploaded the set i have in fusion.
• The cam duration is much longer now, not so efficiënt but much more fun ;)
• added a valve you can print and which works better than the airsoft bullet.
• It needed more clearance around thecrank lobes
• added a 4-cylinder crank
• added a crank stop to put on the outsides
• removed the solid cam follower, it needs a bearing now
  (you can print a plastic 5x2.5mm ring as a replacement for the bearing)
• I've removed the work-in-progress tag. if something is wrong, let me know!

2018-12-13
The single cylinder version was running very well so i made another cylinder and tried the flat twin which ran nicely also.

Things that were done:

• rocker was chancged as it seems to break at 7 bar at which point its running like a madman. (it runs on one cylinder ;))
• cylinder is now a hole all the way through which allows for easy assembly
• cam follower is beefed up as it broke once,
• valve-sring space was made a little tighter.
• added conrods that can be used without bearings. I'm running them at the moment and it seems to work ok.
• added a cam follower without a bearing hole, didn't use it but you can now build it without the smaller ball bearings.

2018-12-11
Today i ran the engine for the first time (single cylinder).
The goal is to test the build and performance.
The following is wrong:

• continuous air leak. not sure if i'ts the ball seal or something else.
• cam opens too short.
• I used a too soft spring which slofs the engine down at higher pressure levels.

in short, it does run, even at 1 bar. I'm running a fairly large 10x5 Master Airscrew. It really needs the weight of the propellor and the engine shakes a lot cause of the torque. I never ran the radial as a single cylinder but it shakes a lot less.
The ball bearing rolling on the cam is a huge succes. I tried it without one first (solid print) and is has a lot of resistance.

next:

• fix the leak
• try a little different cam
• maybe print another cylinder set to make a flat twin (same cam ;))
• video?

2018-12-7 0:39 Updated all STL files.
There is room for a second bearing close to the prop.
Added a prop adapter
Added a m5 air nipple
Changed a lot of small things